As for the proximate cause of N-S packing up and heading home, you almost certainly have it right: they took a big hit from the failure of that contract for the Cal-Car replacements and additions, both financially and to face. They probably have looked at the prospects (hinted at by other posters) for future rail work in the US and decided it isn't worth the hassle. They aren't the only ones to do that, now or in the recent past.

As for SMART's extra cars, they're built, the rest of them seem to work fairly well, and the last ones apparently (another poster said) are on the boat. SMART will have to pay an extra tariff to get delivery, but at this point it's only a little extra - call it sales tax that even government agencies have to pay - though Good Luck getting N-S to eat it (imo of course N-S shouldn't; send the extra bill to FTA). Any expansion to the north will probably not go beyond Healdsburg for a long time - the traffic just isn't there to support reopening the rail line to Cloverdale let alone (based on NCRA rights) beyond. However, the poster who says there's no auto traffic should observe 101 at peak periods - quite crowded from Healdsburg and especially Windsor south. That said, SMART would need to pull off a minor miracle to develop much more traffic, especially past (cute tourist-destination) Healdsburg, than Sonoma Co. Transit hauls on buses now.